Kinetics, isotope effects, and mechanism of the reaction of 1-nitro-1(4-nitrophenyl)alkanes with DBU in acetonitrile

Abstract

The kinetics of the reaction of O,NC,H,C(L)(R)NO, (R = Me, Et, Pri, NNPE, NNPP, or MNNPP, respectively; L = H, D) with 1,8-diazabicyclo[5.4.O]undec-7-ene (DBU) in acetonitrile (MeCN) are reported. The nature of the product indicates that substantial dissociation into free ions occurs. The usefulness of Benesi-Hildebrand relationship for distinguishing between ion pairs and ions of the product is discussed in detail. The reaction shows l o w activation enthalpy value AHS = 15.4, 17.8, and 19.9 kJ mol-' and large negative entropies of activation ASS = -131, -134, and -147 J mol-' K-' for N N PE, N N PP, and M N N PP respectively. The kinetic isotope effects k H / k D (I 2.5, 12.4, and 12.3) are large, showing no variation, the more sterically hindered the substrate. The values of the isotope effects exerted on the activation parameters indicate the contribution of a tunnelling effect QH/QD = 1.35 at 25 "C.The influence of water o n the kinetics is also examined and discussed with respect t o reliability of kinetic measurements of such reactions systems.' 3-1 ho wever the problem is far from resolved. In view of

Kinetics, isotope effects of the reaction of 1-(4-nitrophenyl)-1-nitroalkanes with DBU in tetrahydrofuran and chlorobenzene solvents

“…cular proton transfer reaction gave a dissociated product in MeCN to a large extent, contrary to earlier reports (2)(3)(4)(5). Therefore the applicability of the Benesi-Hildebrand method (6) has been carefully discussed, to explain why it gave incorrect results in previous papers, published by different authors ( 1 ) . We also found large values of the primary deuterium kinetic isotope effect (KIE) despite the high polarity of the solvent, acetonitrile (MeCN), in which the reaction was carried out.…”

“…This suggested that the reactions in polar solvents follow an uncoupled mechanism (19) in which solvent reorganization precedes the proton transfer step, and is supported by the fact that the energy of activation is a complex function in which the part corresponding to the solvation effects in ionogenic reactions usually prevails (20). Taking into account that this idea is inconsistent with previously published results (7,(15)(16)(17)(18), the intention of the present paper is to approach the problem using two different solvents, tetrahydrofuran (THF) and chlorobenzene (CB), for the series of reactions of 1-(4-nitropheny1)-1 -nitroalkanes that have already been studied in MeCN solvent (1). We purposely chose THF and CB solvents because of the dramatic difference in KIE (12.7 and 50) previously found by Caldin et al (18) for similar reactions.…”

“…4.0]undec-7-ene (DBU) has already been examined in acetonitrile (MeCN) solvent (1). We found that this bimole-'~u t h o r to whom correspondence may be addressed.…”

“…However, the most common view has been that high polarity solvents reduced the large isotope effects mainly because of coupling of the motion of the proton with that of the solvent molecules (15,17,18) increasing the effective masses of the transferred hydron. However, our former studies (1,5,(7)(8)(9) provided reason to believe that this reduction is not very large, suggesting that the KIEs are not very sensitive to the polarity of the solvent in which the proton transfer reaction was carried out. This suggested that the reactions in polar solvents follow an uncoupled mechanism (19) in which solvent reorganization precedes the proton transfer step, and is supported by the fact that the energy of activation is a complex function in which the part corresponding to the solvation effects in ionogenic reactions usually prevails (20).…”

Equilibrium and kinetic study of the proton transfer reactions between nitroalkanes and strong organic bases — phosphazenes in tetrahydrofuran solvent

“…These results show that the steric factor and ordering effects in forming the transition state play an important role, confirmed by negative and relatively large entropies of activation: ∆S ‡ = -149.7, -176.5, -178.7, -227.8 J mol -1 K -1 . The reactions between nitroalkanes with different steric hindrance and classic nitrogenous bases such as amidines or guanidines in acetonitrile and tetrahydrofuran have been disscussed previously [12][13][14][15].…”